Bolt operated muzzle-loading firearms have included receivers configured to engage the bolt after the bolt handle is moved forward and turned down. While bolts and their firing units have been placed selected distances from the percussion cap in the breech by so locking the bolt, prior bolt operated muzzle-loading firearms have permitted the bolt to be moved forward against the percussion cap, particularly if the nipple or breech plug biasing the percussion cap is not completely tightened fully into the barrel or receiver of the firearm.